1. Field of the Invention
The invention relates to a burner for gas fuels, in particular natural gas.
2. Description of the Related Art
In most industrial furnaces, for example glassmaking furnaces, the transfer of heat to the charge by radiation plays a dominant role. However, this radiative heat transfer depends, on the one hand, on the emissive power of the combustion products and, on the other hand, on their spatial distribution.
In the glassmaking industry, it has been observed that the manufacture of certain glasses in a furnace, for example technical glasses for liquid-crystal displays (LCDs), which are about 1 mm thick, demands quite particular properties of the flame heating the furnace.
Among these properties, it has been observed that, on the one hand, it is preferable to have a flame which preferably comprises substantially no reducing species, such as soot particles and, on the other hand, to have a flame whose emission range is in the wavelength band between about 2 .mu.m and about 5 .mu.m, that is to say flames which are blue in colour.
Blue flames fulfil these two conditions. They do not produce soot. Their emission is for the most part due, on the one hand, to the emission from water vapour whose favoured emission range is between 2.2 .mu.m and 2.8 .mu.m and, on the other hand, to the emission from carbon dioxide whose emission range is between 2.8 .mu.m and 3 .mu.m, as well as between 4 .mu.m and 5 .mu.m.
Finally, it has been observed that the heat transfer from a flame may be connected with its momentum. Thus, the higher the momentum of a flame is, the further the heat transfer from this flame is shifted longitudinally downstream.
For manufacturing the aforementioned technical glasses, it has also been observed that a flame having limited (or low) momentum is needed, that is to say a flame which is non-hard, and preferably soft, so as not to overheat the refractory walls of the furnace which face the burner.
FR-A-1,441,671, in the name of the Applicant Company, discloses a variable-flame burner for hydrocarbon gases, in particular natural gas. This known burner has a plurality of tubes which are arranged concentrically so that they respectively form a central duct, an intermediate annular duct and a peripheral annular duct. The central duct and the peripheral duct are fed with an oxidant, and the intermediate annular duct is fed with a fuel, or vice versa.
Even though it was possible to adjust the flame of this burner in order to obtain a blue flame, this flame is only obtained with flames having a high momentum, that is to say hard flames.